1. Field of the Invention
This invention relates to a time indicator and, in particular, to a long term time indicator which provides a rapid and clear indication of expiration.
2. Description of the Related Art
Numerous devices are known which provide, after activation, a visual indication of the passage of a predetermined amount of time. Such a time indicator is useful, for example, as a security badge, as an indicator of the length of time a perishable item has been on the wholesaler's or retailer's shelf and for numerous other uses.
A problem that generally exists with such a time indicator is that it gradually changes color over a period of time and it is difficult to ascertain the completion of the time interval. What is required for a long term time indicator is a technology wherein the dye does not begin to appear until the end or near the end of the time interval. Such a time indicator remains unchanged (white or clear) until near the end of the time interval, and then the color (or image) rapidly or, ideally, instantaneously appears. In essence, what is desirable is a time switch (a color-appearing step-function from white to dark), which stays white until the end of the time interval and then produces a step-function, meaning an instantaneous or rapid color change to clearly show that the time interval has ended.
Earlier technologies, are only useful for short time intervals such as days or weeks. They are not useful for longer time intervals such as months because the color change occurs by dye diffusion which begins the instant the activating adhesive cover is applied over these printed dyes. Even though the dye may be very slow in migrating to become visible, it is a problem for the observer in identifying exactly when the image or color indicates the actual time interval has occurred. The time interval for the image appearance, from white to colored due to the dye migration (say 10-20% tint) is proportional to the time interval of the initial time interval for which the time indicator has been designed: in other words, if the time indicator is for two months, the actual color change to be viewed by the observer due to dye migration will be about two months.
Thus, the time indicator stays pure white for about a month; it then starts to change color. In about three months, the observer sees a definite color change of, say 10-20% tint; during the time interval between one and three months, the time indicator is in-between absolutely YES and absolutely NO, a "grey area". This lack of a sharp transition time is the problem with simple dye diffusion systems.
Many of the known time indicators, which are generally short term time indicators are based on the migration of ink from one substrate through another substrate, i.e. in a path perpendicular to the surface of the substrate. After the ink migrates through the substrate(s) it is viewed on a display surface to thereby indicate that the predetermined time has elapsed.
U.S. Pat. No. 4,903,254 to Haas describes a time indicator comprising a number of layers that are adhered together. The indicator has a front part and a rear part. The rear part includes an ink film layer upon a backup layer. At issuance, the front part and rear part are adhered together and the ink migrates from the rear part into the front part.
U.S. Pat. No. 5,364,132 Haas et al. describes a reusable self-expiring security identification badge. When the badge is issued, the inked substrate is attached to the base substrate, the inked substrate covering the void indicia area. The overlay substrate is then placed over and attached with the soluble ink of the ink substrate. The ink dissolver of the overlay substrate contacts and co-acts with the soluble ink of the inked substrate to dissolve the ink and allow the ink to migrate through to the overlay substrate to the display surface, where it can be visually perceived, in a preselected time interval.
U.S. Pat. No. 3,520,124 to Myers, describes a parked car time indicator. The device includes a first sheet having a first reactant and a second sheet having a second reactant and a release sheet which is peeled away to permit contact of the first sheet with the second sheet. Such contact begins a reaction which extends over a selected time interval and terminates with a color change of the device.
U.S. Pat. No. 4,154,107 to Giezen et al. describes a time temperature indicating device having an indicator layer and a signalling component which when in contact with the indicating layer causes the indicator layer to undergo a visually perceptible change.
U.S. Pat. No. 4,212,153 to Kydonieus, et al. describes a laminated indicator which changes in a visually perceptible mode with the passage of time. The indicator comprises at least two layers whereby the molecular migration of an agent in an interior layer to the outermost surface of the exterior layer causes a change which is visually perceptible.
U.S. Pat. No. 5,045,283 to Patel describes a device comprising an activator tape which includes an activator composition and an indicating tape which includes an indicator composition. The tapes are adhesively bound together to form a wedge-shaped composite matrix. The activating composition diffuses through the increasingly thicker composite matrix to contact the indicating composition to produce a visually observable color change at the temperature being monitored. The color change appears as a moving boundary at the color/non-color interface which moves transversely along the length of the device toward the thicker end.
U.S. Pat. No. 5,107,470 to Pedicano describes a quick acting indicator comprising a migrating ink that migrates through an opaque layer to display a message. Upon removal of a release paper, a coated indicator portion is brought into contact with a printed base portion, and a printed message migrates through a non-curing layer to display the printed message.
U.S. Pat. No. 5,317,980 to Muller et al. describes a time indicator comprising chemical substances separated by a barrier. When the barrier is corroded or removed the chemicals act to cause a visible color change.
Other known time indicators are based on the migration of liquids or jelly through wicks to indicate the passage of time.
For example, U.S. Pat. Nos. 3,954,011 and 3,962,920 to Manske describe a time indicating device which includes a porous fluid-carrying pad, a wick material and an indicator means whereby the progress of fluid along the wick material can be visibly indicated and used to measure the passage of time, the exposure to a given minimum temperature or time-temperature relationship.
U.S. Pat. No. 4,028,876 to Delatorre describes an apparatus for visually indicating elapsed time by a color change which comprises a transparent flexible container having a rupturable sealed capsule therein containing a chromophoric compound. The capsule when ruptured permeates into another capsule contained within the container to combine with another chromophoric compound to form a colored body after the predetermined time has elapsed.
U.S. Pat. No. 4,229,813 to Lilly, et al. describes a time indicator which utilizes an ampule reservoir of silicon oil which, after crushing of the ampule, releases the silicon oil which is slowly absorbed onto and moves up a porous strip at a rate which is a function of time. One side of the strip is printed with an oil soluble ink, while the other side is unprinted. The printed side of the strip is laminated to an unprinted strip. As the silicon oil moves up the strip, the oil contacts the ink causing a dye in the ink to migrate from the printed side to the unprinted side, thus providing a measurable color front moving up the strip.
U.S. Pat. No. 4,382,700 to Youngren describes an indicator which contains a capsule of mineral jelly which is in contact with a wick, such that the mineral jelly diffuses into the wick in accordance with the changes in ambient temperature over a period of time.
U.S. Pat. Nos. 4,292,916 and 4,408,557 to Bradley, et al. describe, in several embodiments, a time indicator wherein a migrating carrier mixture is caused to flow by rupturing or depressing a capsule. The carrier mixture contacts an absorptive layer and migrates along the absorptive layer. The progress of this migration can be observed. Of particular relevance to the invention described and claimed herein is the embodiment associated with FIG. 9 (col.6).
U.S. Pat. No. 4,432,656 to Allmendinger describes a time/temperature integrator for indicating the history of a deep frozen product through diffusion of water along a cellulose wick.
U.S. Pat. No. 4,629,330 to Nichols describes a color change indicator of time and temperature. The device includes a reservoir of liquid having a predetermined index of refraction and a rate of evaporation. The liquid is covered by an opacifying layer of microporous material. The opacifying layer has an index of refraction approximately the same as that of a liquid and has an open cell network of pores for absorbing liquid from the reservoir. The layer is in a first radiation scattering condition when the liquid occupies the layer and after a specified period of time in a second scattering condition when the liquid is depleted from the layer.
U.S. Pat. No. 4,643,122 to Seybold describes a diffusion controlled security tag comprising a carrier containing a solution of a compound which changes color upon diffusion or evaporation of the solvent. The carrier is enveloped in a barrier film which controls the rate of diffusion/evaporation of the solvent from the carrier, such that a change in color of the carrier indicates undesirable storage or product tampering.
U.S. Pat. No. 5,058,088 to Haas describes a timing indicator type badge, label or display wherein the relative amount of time that has elapsed from the initial activation of the timing indicator can be easily determined by the progression of a visually perceptible change in color along different areas of the timing indicator. This timing indicator comprises a clear self-adhesive film which is placed over a printed substrate in order to activate the timing indicator. The printed substrate includes a migrating or soluble ink which migrates along the substrate to produce a visual color change.
PCT/AU90/00433 to Tothill et al. discloses a timer comprising a lamination containing a piece of porous wick material and a reservoir of liquid. The reservoir of liquid soaks into the porous wick to create a visible trace of the liquid on the porous wick. A face part of the lamination is transparent so that the visible trace can be seen.
U.S. Pat. No. 3,243,303 to Johnson discloses a temperature monitor employing a flowable aqueous composition containing dispersed polyvinyl acetate as a flow retardant. The fluid carrier is impregnated with a fluid composition which includes a coloring material. Initially the fluid composition is immobilized by freezing. As the composition is absorbed by an absorbent layer. An impermeable layer serves as a barrier but permits the fluid carrier to pass around the ends thereof to reach an indicating area. A masking layer obscures the absorbent layer and the color change taking place therein until the food reaches a central transverse opening in the masking layer, which opening registers would be an indicating layer.
U.S. Pat. No. 3,046,786 to Tessem discloses a condition responsive device arranged to indicate the extent of exposure above a certain minimum temperature point for use in connection with frozen foods. The device includes a galvanic cell parallel a porous medium impregnated with an electrolyte and which includes an indicator such as phenolphthalein which has one color in the presence of the electrolyte and which will change color an electrolytic action that is temperature dependant.
U.S. Pat. No. 2,896,568 to Pryor et al. discloses a temperature indicating device comprising substances introduced in liquid form into wells and frozen in situ, which melt and migrate out of the wells as a result of gravity or capillaries may be detected by observation.
Other indicators in the prior art rely primarily upon chemical reactions to cause a visually perceptible change over a desired time period rather than the migration of fluids or compounds. These include, for example:
U.S. Pat. No. 2,337,534 to Barber describes a magazine page exposure time indicator including a photosensitive paper sheet mounted on a magazine page, and a developed photographic film sheet having a series of adjacent portions of varying density mounted over the photosensitive paper sheet.
U.S. Pat. No. 3,018,611 to Biritz describes a time indicator including an oxygen reactive material which reacts and changes color upon exposure to oxygen.
U.S. Pat. No. 3,480,402 to Jackson describes a time indicator formed of an absorbent carrier having absorbed thereon at least one chemical compound which changes color upon exposure to oxygen.
U.S. Pat. No. 3,999,946 to Patel et al. describes a perishable product time-temperature history indicator which includes a substrate for attachment of the indicator to a product. The device includes compositions containing at least two conjugated acetylene groups which exhibit sequences of irreversible color changes at combinations of time and temperatures.
U.S. Pat. No. 4,195,058 to Patel describes a device for monitoring time-temperature histories in which a vapor is allowed to permeate through a permeable vapor to contact a liquid polydiacetylene indicator to provide a color response after a predetermined period of time.
U.S. Pat. No. 4,212,393 to Lenkoff describes "magic" pictures printed with a water soluble ink including therein one or more latent water soluble coloring materials which may be printed in a pattern of dots. When the soluble imprints of ink are contacted by a felt tip pen, the water in the pen is deposited on the paper and releases the latent coloring material thereby spreading the color along the picture.
U.S. Pat. Nos. 4,432,630, 4,542,982 and 4,779,120 to Haas describe badges with an ambient light sensitive coating thereon which when exposed to ambient light for a specified period of time change to a specified color.
U.S. Pat. Nos. 4,812,053 and 4,917,503 to Bhattacharjee et al. describe, respectively, an oxygen-sensitive and light sensitive perishable product time-temperature indicator.
U.S. Pat. No. 4,987,849 to Sherman discloses a signal device comprising two inks of like colors; an ink that is stable in the presence of light and/or air and an ink that is sensitive to light and/or air. The stable ink is applied to the device in the form of a message. The sensitive ink is applied as a background to the stable ink to camouflage the message. The device is activated by exposing it to light and/or air, whereby the sensitive ink fades and a message is left behind.
Other possibly relevant prior art includes: U.S. Pat. No. 1,454,837 to Smith for tamper evident paper; U.S. Pat. No. 2,088,567 to Ballou for a tamper evident identification card; U.S. Pat. No. 2,780,015 to Whitehead for tamper evident identification cards; U.S. Pat. No. 3,078,182 to Crone, Jr., et al. for a color changing pressure sensitive adhesive indicator tape; U.S. Pat. No. 3,311,084 to Eidenbaum for a pressure sensitive adhesive tape containing markings which change color when subjected to steam sterilizing conditions; U.S. Pat. No. 3,921,318 to Calavetta for medical history cards; U.S. Pat. No. 4,382,063 to Romito et al. for a sterility indicator device having ink spots of chromium chloride that change color over a period of time in the presence of steam; U.S. Pat. No. 4,404,922 to Morane et al. for an aging indicator having at least one closed tube of liquid which, as a function of time and temperature, gradually permeates out of the tube, the level of fluid in the tube being a measure of aging; U.S. Pat. No. 4,573,711 to Hyde for a secure credit card; U.S. Pat. No. 4,643,588 to Postle et al. for a method of monitoring if a stored product has exceeded a predetermined temperature; U.S. Pat. No. 4,646,066 to Baughman for an environmental exposure indicator which includes a tuned electrical circuit; U.S. Pat. No. 4,846,095 to Emslander for a device for indicating a temperature is above/below a critical temperature; U.S. Pat. No. 4,846,502 to Chang et al. for tamper evident documents; U.S. Pat. No. 5,293,648 to Finley for a tag for visually indicating loss of a protective agent; and U.S. Pat. No. 5,378,430 to Nieves et al. for a steam sterilization process monitor.
All of the aforedescribed devices are complicated to adjust for a selected period of time. Adjustments often involve experimentation with many types of chemicals, inks, solvents, wicks, etc. to prepare a device which can operate under the conditions expected. Most of the prior art devices gradually change color over a period of time and involve, at best, a guess on how much time has elapsed. When this is combined with the possible variations in temperature, humidity, etc. that may exist in the environment of the time indicator, the viewer has very little confidence that he is close to the expiration time of the device.